Twilight Factor

[Sakoman]

Whenlooking at Optics how do you tell what is good and what is a poor twilight factor??

Thanks.

 

[AP]

17 or better where the exit pupil is 5mm or bigger is good...

 

[Dogleg]

The "Twilight factor" is a basically useless number, except maybe for Ad-men and salesmen. It is arrived at by multipling the objective diameter in millimeters by the power, then taking the square root of that. A 3-9 x40 gets a Twilight factor of 18.97 (OK 19) whether the scope in question is a BSA, or a Swarovski. Does anyone think these scopes are equal? Lens quality and coatings are the biggest factors in brightness, but they aren't considered in the formula.